Method and apparatus for time synchronisation in wireless networks

ABSTRACT

A wireless media distribution system is provided comprising an access point ( 6 ) for broadcasting media and a plurality of stations ( 2 ) for reception and playback of media. Each station is configured for receiving and decoding a timestamp in a beacon frame transmitted repeatedly from the access point. This is used to control the output signal of a station physical layer clock ( 12 ) which is then used as a clock source for an application layer time synchronization protocol. This application layer time synchronization protocol can then be used in the station to control an operating system clock ( 8 ) for regulating playback of media.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from GB application 1209312.6, filed onMay 24, 2012, and from GB application 1116521.4, filed on Sep. 23, 2011,both of which are incorporated herein in their entireties for allpurposes.

BACKGROUND

1. Field

This invention relates to methods and apparatus and systems for timesynchronisation in wireless networks and in particular to wirelessnetworks of the type where time synchronisation is important to obtainacceptable performance from receiving units in a wireless network, suchas in audio and video functions and other multi-media functions.

2. Related Art

The IEEE 802.11 Wi-Fi standard is commonly used for many wirelessapplications between computing devices, network servers and the like. Itis increasingly being used in domestic applications for functions suchas streaming audio data, streaming video data, and other multi-mediaapplications such as gaming using portable gaming devices. Data which isto be transmitted to a plurality of stations (STA) in a wireless networkis provided via an access point (AP). This broadcasts the relevant dataand it is detected by one or more receivers or stations for which it isintended.

Each station in the network will have circuitry associated with it toprocess the data in accordance with the type of data received. Forexample, a pair of wireless speakers will each receive left and rightchannels of an audio signal and will have to decode this and play itback for a user to listen to. The circuitry in each station will have afree running clock. This is usually a crystal based oscillator and it iscommon for these not to be particularly accurate, and they are thereforeoften not synchronised with each other between stations. Thus, for e.g.a pair of loud speakers the free running clock used to control playbackmay be faster in one loud speaker than in the other which will graduallycause the left and right channels to become less synchronised.

Time protocols such as NTP, e.g. NTPRFC1305, are commonly used toprovide a strict timing mechanism over standard wired distributionsystems e.g. Internet wired networks.

SUMMARY

NTP is such a commonly used protocol, it is desirable to use it whentransmitting data in a wireless network for many applications, such asaudio. However, a problem arises because a time protocol such as NTPwithin a wireless system experiences asymmetric receive and transmitpaths which can differ as to the degree of asymmetry between differentunits in the system. Therefore, a station using a time protocol such asNTP via a Wi-Fi network will suffer from inaccuracies due to theasymmetric paths and hence may not be synchronised with other stations.

We have appreciated that Wi-Fi transmission standards such as IEEE802.11include a timing beacon (time synchronisation function—TSF) provided bya physical layer clock at an Access Point which is required to beprocessed by a receiver as a priority, and furthermore this forms a fastand direct half-symmetrical path between the AP and each station. TheWi-Fi receiver in each station has its own physical layer free runningclock, used by the processing circuitry at the station. This is reset tosynchronise with the timing beacon from an A Pin a received Wi-Fisignal. Thus, the physical layer clocks in the receivers can besynchronised using the TSF. The difference between the received AP-TSFand the STA free running clock in the Wi-Fi receiver provides an errorbetween the AP clock and the STA station clock and this can be used toprovide a finer correction value for a higher level time synchronisationmethod (such as NTP), such that the station clocks in a plurality ofstations are more closely synchronised.

In one aspect, there is provided a wireless station for reception andplayback of media transmitted in a wireless media distribution system.The station has a physical layer which includes a physical layer clockand further includes receiver and decoder circuitry for repeatedlyreceiving and decoding a broadcast beacon frame and a timestamp includedtherein. The physical layer clock is configured to control its outputclock signal in a dependence on the timestamp. A transport layeroperates using a transport layer time synchronisation protocol andincludes a clock interface connected to the physical layer clock toconvert a clock signal from the physical layer clock to a form suitablefor use as a clock source by the transport layer time synchronisationprotocol. An operating system clock is provided and controlled by thetransport layer time synchronisation protocol for reception and playbackof media.

In another aspect, there is provided a method for controlling anoperating system clock in a wireless station for reception and playbackof media transmitted in a wireless media distribution system. The methodcomprises receiving a broadcast beacon frame in a physical layer of thestation and decoding the beacon frame to derive a timestamp. An outputclock signal of a physical layer clock is controlled in dependence onthe timestamp. The output clock signal of the physical layer clock isconverted to a form suitable for use as a clock source by a transportlayer time synchronisation protocol in a transport layer. An operatingsystem clock is controlled for media reception and playback with thetransport layer time synchronisation protocol.

In a further aspect, there is provided a wireless media distributionsystem with an access point for broadcasting media and a plurality ofstations for reception and playback of media. The access point isconfigured to transmit repeatedly a beacon frame which includes atimestamp derived from a physical layer clock in the access point. Eachstation is configured to receive and decode the timestamp in each beaconframe and to control an output clock signal of a station physical layerclock in dependence on the timestamp and to use the output clock signalof the physical layer clock as a clock source for a transport layer timesynchronisation protocol in a transport layer of the station. Theseexemplary aspects allow a variety of embodiments to be realizedaccording to their disclosures, which are defined with more precision inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

An example will now be described in detail by way of example withreference to the accompanying drawings in which:

FIG. 1 shows schematically a Wi-Fi network of the type to which theinvention may be applied, which uses NTP as an example time protocol.

FIG. 2 shows the arrangement of FIG. 1 with physical layer clocks at theAP and STAs being used for synchronisation;

FIG. 3 shows a schematic diagram of an AP physical layer clock andassociated circuitry;

FIG. 4 shows a first embodiment of an STA physical layer clock andassociated circuitry;

FIG. 5 shows a second embodiment of an STA physical layer clock andassociated circuitry; and

FIG. 6 shows the relationship between a physical layer clock and an NTPclock and circuitry in the application layer of a Wi-Fi system.

DETAILED DESCRIPTION

FIG. 1 illustrates the known technique for time-synchronising twowireless stations using an application layer protocol such as NTP. Inthis example, there are two stations (STA) 2 that are playing back mediafrom a media source 4. The media source can be located in the internet(e.g. a streaming service), connected to an AP 6 (e.g. stored on a localcomputer or network drive), or can derive from one of the stations (e.g.the media may be broadcast via DAB and received at one station equippedwith a DAB receiver and provided to the other station over Wi-Fi). Thetwo stations aim to playback the media in sync. This can comprise eitherplaying back the same media from the two stations in sync (e.g. formulti-room playback) or playing different components of the media insync (such as left and right stereo channels provided to different loudspeakers).

The playback of the media is controlled by an operating system (OS)software clock 8 in each station 2, and the two stations can be arrangedto start playback at a specific time, with the rate of playbackdetermined by the respective OS clock 8. However, this relies on the OSsoftware clocks in the stations being synchronised with each other.Generally, this is achieved using NTP (although other similar timeprotocols can be used). NTP is an application layer time synchronisationprotocol, which can be used to control the OS software clock at eachstation. With NTP, each station periodically polls a time server 10accessible over the internet (typically an atomic clock server). Thisreturns a time value, and NTP can use this in combination with othermeasured network statistics to predict a “real” time value. In an idealnetwork environment (such as wired Ethernet LAN), NTP can give goodsynchronisation (˜1 ms accuracy), which is adequate for multi roomplayback.

However, there are problems with using an NTP-based solution such asthat described above in a wireless environment such as Wi-Fi.Specifically, the asymmetric receive and transmit paths can result in aninaccurate prediction being applied by NTP, and hence the OS softwareclocks in each station will not be adequately synchronised, as discussedabove. Furthermore, for applications such as left/right stereo channelsynchronisation, even higher accuracy is needed (around 10 us) which NTPcannot usually achieve. Therefore, the above type of NTP-based system isnot suitable, particularly for left/right channel synchronisation butalso for some multi room applications.

Alternative prior art solutions have moved away from NTP entirely, butthese bring their own problems as they usually use a proprietarysynchronisation protocol. This results in non-standard STAs being used(as generally one must act as a master to sync with the others). Inaddition, it precludes any mixed systems where some stations arewireless and others are connected to a wired network.

Embodiments of the present invention retain the use of the NTPapplication layer time synchronisation protocol to synchronise the OSclock 8 at each station 2. However, in this instance, instead of NTPpolling a remote time server 10 to synchronise clocks, the OS softwareclock in each station 2 uses a physical layer clock 12 at that stationas a time source. Furthermore, the time source that NTP uses isswitchable. In some cases where the NTP accuracy is adequate using aninternet based timeserver 10 (e.g. many multi room systems or wherethere are mixed wired and wireless stations). In other instances, wherehigher accuracy is required, the NTP time source is switched to a moreaccurate source which is in this case is the physical layer clock usedto control wireless communication in each station and in the accesspoint. Each station and access point has its own physical layer clock.The physical layer clock is used in e.g. IEEE802.11 for clocking framedata rates in the Wi-Fi system.

This is illustrated with reference to FIG. 2 in which the OS softwareclock 8 at each station polls a physical layer clock 12 at each stationusing NTP such that the physical layer clock 12 becomes the source ofsynchronisation for the OS software clock 8 at each station. Thephysical layer clock 12 at each station is further synchronised to aphysical layer clock 14 at an access point 6.

FIG. 2 illustrates this arrangement in which NTP uses the physical layerclock 12 at each station as its timing source, and in this instance whenthe system is Wi-Fi based, a time synchronisation function is exchangedwith the physical layer clock 14 at the access point 6 so as tosubstantially synchronise timings at two or more stations 2. As in FIG.1 the media to be played back can be provided via a streaming servicefrom the internet, via a unit connected to an access point 6, or,alternatively, can be embedded at one of the stations.

It will be appreciated that using such an arrangement each stationindependently synchronises its OS software clock to its own physicallayer clock using NTP. This synchronisation is distributed amongst thestations and therefore no predefined hierarchy is required.

In the case of Wi-Fi systems such as that shown in FIG. 2 the physicallayer clock 12 at each station is kept in synchronisation using a timingsynchronisation function (TSF). This comprises the periodic creation ofbeacon frames and transmission of these using RF. This beacon frame isbroadcast by the AP and therefore received by all stations.

FIG. 3 shows schematically the operation of a physical layer clock 14 atan access point 6. This comprises a free running local oscillator (e.g.a local crystal oscillator) 16 which generates a clocking signal and acounter arranged to increment at each clock “tick” or clock cycle. Aframing unit 20 periodically creates beacon frames and transmits thisusing RF front-end circuitry 22 coupled to a Wi-Fi antenna 24. Eachbeacon frame, which is broadcast and received by all stations 2comprises a time stamp field which contains a copy of the counter valuefrom the AP counter 18. This feature of transmitting beacon frames ispart of the Wi-Fi standard (IEEE802.11). The transmission of the beaconframe is fast and direct and all stations receive the framesubstantially simultaneously. The beacon frame and TSF is typicallybroadcast every 100 ms or so.

In order to fully understand how this embodiment of the inventionoperates it is necessary to understand the structure of the physicallayer clock in each station 2. This is shown in FIG. 4.

In a manner similar to that at the access point 6, the station 2 has afree running oscillator formed from a local crystal 28. A counter 30 isincremented on each clock cycle at each clock “tick” as happens at theaccess point. The intention is that the free running oscillator 28 ineach station should produce a clock signal with the same frequency asthe free running oscillator at the access point. However, there arevariations between individual crystals used in each oscillator and as aresult of these they run at slightly different rates. Because of thisthe counters at the access point and at each of the stations will beincremented at slightly different rates and even if periodicallysynchronised will drift over time between synchronisations.

To deal with this frequency variation and drift, when a station receivesa beacon frame via its RF front-end 32, the time stamp field isextracted by a decoding block 34. This time stamp field includes thecounter value of the counter at the access point. This counter valuefrom the access point is then copied to the counter 30 in the station.Thus, each time a beacon frame and the time stamp field are received,the counter 30 at the station is resynchronised to the value of thecounter 18 at the access point 6. As the beacon frame is transmitted atRF frequencies, the time delay involved in its transmission isinsignificant. Furthermore, the beacon frame and time stamp are a lowlevel function within the Wi-Fi standard and are not subject tosignificant processing delays which would require further compensation,and therefore insignificant delay is applied.

The physical layer clock 18 and its equivalent at the access pointcomprises the crystal oscillator and the counter which is a monotonichardware counter formed in silicon. In a Wi-Fi chip it is this counterthat is used by the whole chip as a low latency counter available to theupper processing layers as a common clock signal. In one example theclock may comprise a 24 bit counter ticking at 192 KHz, but other bitdepths and frequencies are possible, and these are parsed to form anappropriate input to NTP.

In the embodiment illustrated here, the station physical layer clockalso comprises additional functionality which is used to rate-controlthe counter 30. This is provided to align the station counter 30 withthe access point counter 18 as closely as possible such that thediversion between beacon frames is minimised and may be eliminatedaltogether. In this example, the station physical layer clock shown inFIG. 4 has a rate adjustment module 36 coupled between the local crystaloscillator 28 and the counter 30. This enables the rate at which thecounter 30 is incremented by the crystal to be adjusted. For example,the rate adjustment module may be a software module controlling the tickrate provided to the counter 30. In another example, the rate adjustmentmodule may be a hardware module such as a phase locked loop used tocontrol the tick rate supplied to the counter 30. The rate adjustmentmodule 36 is used to adjust the rate at which the clock signalincrements the counter 30 to match, as closely as possible, the rate atwhich the access point local crystal oscillator 16 increments the accesspoint counter 18. This is achieved by using a time stamp recordal anderror calculation unit 38. This receives the time stamp from eachdecoded beacon frame and calculates the difference between the accesspoint counter value contained in the time stamp and a current countervalue from the station counter 30. This error is calculated before thestation counter 30 is overwritten with the access point counter value.This may be achieved by using the time stamp and error calculating unitto repeatedly monitor the value of the counter 30 by updating amonitored value substantially simultaneously each time the counter 30 isupdated. Alternatively, the timestamp and error calculation can countthe number of clock ticks between arrival of the beacon frame and theSTA counter being read and can correct the clock signals accordingly.

The time stamp and error calculating unit 38 therefore provides an errorvalue between the access point and station counters 18 and 30 whichrepresents the difference between them or the difference by which theyhave drifted since reception of the last beacon frame.

The error value thus derived can then be provided to a rate correctioncalculation module 40. This is used to determine how to control the rateadjustment unit 36 to adjust the station counter for the derived error.One way in which this can be achieved is through knowledge of the timeelapsed since a previous correction and the current error value. Thisinformation, in combination with the error value enables the amount ofdrift between the access point counter and the station counter to bedetermined. The clock signal from the crystal can then be adjusted inthe rate adjustment unit 36 to increment the station counter 30 at arate much closer to the rate of the access point counter 18. Becausethis happens repeatedly at the reception of each time stamp, the errorshould tend towards a minimum over a relatively short period of time andusing sufficiently accurate error calculation and rate adjustmentcircuitry will enable close alignment of access point and stationcounters 18 and 30.

The same technique is performed independently in each station 2 by usingthe respective physical layer clock in each station. This will result ina counter at each station which is closely synchronised to the accesspoint counter and also to each other station counter.

The physical layer clock correction technique described above is notpart of the Wi-Fi standard. However, we have appreciated that low levelaccess to the Wi-Fi drivers can be made and this enables access to thereceived beacon frames and counter values. Thus, these have been used toobtain much closer synchronisation between access points and stations.

The above embodiment could be implemented without using the errorcalculation and rate adjustment. However, there would be greater driftbetween local oscillators between reception of beacon frames and thesame drift would repeat and could not be reduced using the rateadjustment and clock correction module.

Another example of the physical layer clock in each station is shown inFIG. 5. As in FIG. 4 this comprises a local crystal oscillator 28 and acounter 30. The RF front-end circuitry 32 and decoding unit 34 receivesbeacon frames and from these decode and update the station counter 30using the access point counter value from the beacon frames.

In this example, instead of rate controlling the station counter 30, thephysical layer clock generates a pair of values corresponding to theaccess point counter and the station counter at the time at which theaccess point counter is received. These are generated in a time stampand pair generation unit 42. This is referred to as a correlated accesspoint and station counter pair. To achieve this, the arrival time of theaccess point counter is time stamped by the station counter 30 and thestation counter value then read/derived from that time stamp. Once thecorrelated counter pair has been generated it can be passed to highersoftware layers for processing. These higher layers can performcompensation of the rate difference between the station crystal and theaccess point crystal 28. For example, this can be performed by an NTPclock interface as will be described below. Because the access point andstation counter pair are correlated to the same time stamp, it does notmatter if the higher layers take some time to process the pair andperform the compensation.

In this example, the higher software layers can use the correlatedcounter pair to monitor how much the station counter diverges from theaccess point counter over the course of the time period between beaconframes being received at a station. Preferably, this is monitored overseveral beacon frames to improve the accuracy of this divergence ordrift. Once this drift is known a higher software layer can ratecompensate the station counter value by determining the time since thelast beacon frame and calculating how much the station counter will havediverged in that period and adjusting the current counter valueaccordingly.

By using either of the embodiments of the physical layer clock describedwith reference to FIGS. 4 and 5 above each station 2 has a counter 30with much improved synchronisation to the access point counter 18. Ifimplemented with sufficient accuracy, the station and access pointcounters will be substantially synchronised.

However, the counter in a physical layer clock is not in itself suitablefor use with NTP. Because of this, an NTP clock interface in theapplication layer of the Wi-Fi system is required as shown in FIG. 6.This interface 50 takes a current counter value from the physical layerclock 52. If used in the second example described above with referenceto FIG. 5 this may be rate compensated to adjust for any drift betweenthe reception of beacon frames. The NTP clock interface parses thephysical layer clock such that it is in a form suitable for use by NTP,that is to say it is placed in a similar form to that provided byinternet clock servers. For the purpose of synchronising media playbackbetween stations it does not matter if the NTP clock interface providesa correct time of day but what is important is that all the stations andtheir clocks are synchronised to the same time.

An NTP source selector 54 is configured to select the NTP clock fromeither the physical layer clock via the NTP clock interface 50 or fromone or more internet based clock sources. This selection can be basedupon the level of accuracy required or on the configuration of theoverall system, or on other factors as appropriate. After this, NTPoperates in the same way as it would in a known wired networkenvironment. NTP periodically polls the clock source and the NTP sourceselected provides the clock values from the selected source to NTPaccuracy predictor 56 which applies standard NTP prediction algorithmsbased on the round trip time (RTT) for the clock polling. In one examplethe algorithm can remain unchanged even though the clock source may beinternal to the station (very short RTT). In other examples thealgorithm can be optimised to take into account the local nature of theclock source. NTP adjustments are made to the operating system softwareclock 8 in a station based on the clock source values and theprediction.

This same process is carried out in all stations. Because of thesynchronisation between the physical layer clocks achieved using thecircuitry of FIG. 4 or 5, synchronised operating system software clocksare obtained in each station. Therefore, media playback on the differentstations is synchronised using the thus synchronised operating systemsoftware clocks such that media is heard (in the case of audio) on astation in synchronisation which each other station.

Further enhancements to the above described system can use an additionalfeature of Wi-Fi to provide additional information to each of thestations on the state of synchronisation. The Wi-Fi standard includes a“probe request” frame which is sent from the stations to the accesspoint to provide information on (amongst other things) supported datarates. However, the probe request frame also has a vendor specific fieldthat can be used for other information. For example, the most recentlycalculated error value showing the difference between the access pointcounter and the station counter can be included in the field. Althoughthe frame is primarily sent to the access point it is actually broadcastover the Wi-Fi network which means that all the other stations are ableto receive it as well.

One or more of the stations can collect information on the error valuesfrom the other stations and this can be used to improve systemperformance, for example, it can be used as an input to the NTPselector. In this case, if the errors are all sufficiently low then itcan switch to the TSF-based clock source or conversely if an STA has ahigh error due to poor reception, then an internet based source may bemore reliable and should be selected by the NTP selector. In anotherexample, this can be used to determine whether the errors are low enoughto enable separate left and right stereo channels though differentstations. If the result of this determination is that separate left andright channels will not be sufficiently synchronised then the system maybe controlled to be revert to the same media (audio) playback througheach channel, that is to say without any stereo separation. In a furtherexample the information can be provided to the user to help the usersite the STA's such that sufficient accuracy is achieved to enablestereo channels to be output. This is particularly useful inenvironments where there is significant multi path distortion.

I claim:
 1. A wireless station for reception and playback of mediatransmitted in a wireless media distribution system, the stationcomprising: a physical layer which includes a physical layer clock andfurther includes receiver and decoder circuitry for repeatedly receivingand decoding a broadcast beacon frame and a timestamp included thereinand wherein the physical layer clock is configured to control its outputclock signal in dependence on the timestamp; an application layer whichoperates using an application layer time synchronisation protocol andincludes a clock interface connected to the physical layer clock toconvert a clock signal from the physical layer clock to a form suitablefor use as a clock source by the application layer time synchronisationprotocol; and an operating system clock controlled by the applicationlayer time synchronisation protocol for regulating playback of media. 2.A wireless station according to claim 1, in which the application layerincludes an application layer time synchronisation protocol sourceselector which is operable to select between the clock interfaceconnected to the physical layer clock and an external clock source.
 3. Awireless station according to claim 2, in which the external clocksource is an internet based clock source.
 4. A wireless stationaccording to claim 2, in which error values are used to control theapplication layer time synchronisation protocol source selector.
 5. Awireless station according to claim 1, in which the physical layer clockcomprises a crystal oscillator and a counter and the output clock signalis controlled by overwriting a value in the counter with a value in thereceived timestamp.
 6. A wireless station according to claim 5, in whichthe physical layer clock further includes rate control circuitry todetermine an error between a current physical layer clock counter valueand a received value in the timestamp and to adjust a rate at which thephysical layer clock is incremented in dependence on said error.
 7. Awireless station according to claim 6, in which the rate controlcircuitry comprises an error calculation unit receiving the timestampand a current physical clock counter value and deriving the error therebetween, a rate correction calculation unit receiving the error andderiving a rate correction in dependence thereon, and a rate adjustmentunit coupled between the crystal oscillator and the counter andreceiving the rate correction unit output and adjusting the rate atwhich the counter increments in dependence thereon.
 8. A wirelessstation according to claim 6, in which the station receives error valuesfrom other stations and is configured to determine from theseimprovements to system performance.
 9. A wireless station according toclaim 8, in which the station is configured to playback one of a pair ofstereo channels and the error values are used to determine thesynchronisation between the pair of stereo channels.
 10. A wirelessstation according to claim 1, in which the physical layer is configuredto generate a pair of values from the physical layer clock and areceived timestamp at the time at which the timestamp is received, in atimestamp and pair generation unit and the application layer includes aninterface to receive the pair of values and is configured to compensateits application layer time synchronisation protocol in dependencethereon.
 11. A wireless station according to claim 1, in which thewireless station uses a Wi-Fi transmission standard.
 12. A wirelessstation according to claim 1, in which the application layer usesNetwork Time Protocol (NTP) as its time synchronisation protocol.
 13. Awireless station according to claim 1, in which a media source iscoupled to the station.
 14. A wireless station according to claim 1, inwhich a media source is accessed via the internet.
 15. A method forcontrolling an operating system clock in a wireless station forreception and playback of media transmitted in a wireless mediadistribution system comprising: receiving a broadcast beacon frame in aphysical layer of the station; decoding the beacon frame to derive atimestamp; controlling an output clock signal of a physical layer clockin dependence on the timestamp; converting the output clock signal ofthe physical layer clock to a form suitable for use as a clock source byan application layer time synchronisation protocol in an applicationlayer; and controlling an operating system clock for regulating mediaplayback with the application layer time synchronisation protocol.
 16. Amethod according to claim 15, further comprising selecting between thephysical layer clock as a clock source for the application layer timesynchronisation protocol and an external clock source.
 17. A methodaccording to claim 16, in which the external clock source is an internetbased clock source.
 18. A method according to claim 15, in whichcontrolling the output clock signal of a physical layer clock comprisesoverwriting a counter value in the physical layer clock with a value inthe timestamp.
 19. A method according to claim 18, further comprisingdetermining an error between a current physical layer clock countervalue and a value in the timestamp, and adjusting a rate at which thephysical layer clock is incremented in dependence on the error.
 20. Amethod according to claim 19, further comprising deriving a ratecorrection from the error and adjusting the rate at which the counter isincremented in dependence thereon.
 21. A method according to claim 15,further comprising generating a pair of values from the physical layerclock and a received timestamp at the time at which the timestamp isreceived, providing the pair of values to an interface to theapplication layer, and compensating the application layer timesynchronisation protocol in dependence thereon.
 22. A method accordingto claim 19, further comprising receiving error values from otherstations and determining from these improvements to system performance.23. A wireless media distribution system comprising: an access point forbroadcasting media and a plurality of stations for reception andplayback of media, wherein: the access point is configured to transmitrepeatedly a beacon frame which includes a timestamp derived from aphysical layer clock in the access point, and each station is configuredto receive and decode the timestamp in each beacon frame and to controlan output clock signal of a station physical layer clock in dependenceon the timestamp and to use the output clock signal of the physicallayer clock as a clock source for an application layer timesynchronisation protocol in an application layer of the station, whereinthe application layer time synchronisation protocol controls anoperating system clock for regulating media playback.